Active flow control is useful for various applications. For example, in propulsion, low-pressure turbines use highly loaded airfoils to improve efficiency and to reduce the number of blades required. The boundary layers, which are fully turbulent at take off, become transitional at high altitudes because of the change in operating environment at cruise. Under such off-design conditions, separation can occur and performance may be impaired. Similarly, in external aerodynamics, the maximum lift and stall characteristics of a wing affect take off and landing distance, maximum and sustained turn rates, climb and glide rates and flight ceiling.
Thus, an efficient device is desired to mitigate performance deficiencies encountered in many practical applications at off-design conditions. Control of subsonic flows employing plasma-induced body forces is currently a topic of considerable interest. This is motivated by several distinct advantages associated with plasma actuators, including the absence of complicated mechanical or pneumatic systems and their moving parts, the absence of mass injection and thus mechanical inertia, their potential for near-instantaneous deployment and operation over a broad range of frequencies, their relatively low power consumption, rapid switch on/off capabilities, and the potential of electromagnetic forces to apply large forces in a relatively precise manner. Additionally, plasma-based devices are capable of operating at atmospheric conditions without uncontrolled macroscopic breakdown.
Recent experiments and numerical studies have successfully demonstrated striking flow control effects of radio frequency (RF) induced dielectric barrier discharge (DBD) at low speeds.
Low-speed flow control typically employs RF dielectric barrier discharge (DBD) to generate a near-surface body force that can reattach separated flows through an induced wall-jet. FIGS. 1 and 2 show schematics of RF induced atmospheric glow discharge for paraelectric (in FIG. 1) and peristaltic (in FIG. 2) flow acceleration at low speed. Surface discharge is the focus of the design shown in FIG. 2, in which one dielectric coated electrode is typically exposed to the flow surface, while the other is grounded and embedded in a layer of insulator and displaced a short streamwise distance from it. FIGS. 1 and 2 illustrate typical arrangements of a monolayer design. An RF voltage is applied to the electrode exposed to the gas. The electric field generated by the discharge is due to the geometric asymmetry as well as the vastly disparate mobility of the electrons and ions. The plasma at this pressure is highly collisional, causing an efficient energy exchange between charged and neutral species. The net forces generated by the intermittent discharge induce ion-“wind,” while ion-neutral collision transfer mechanisms generate the desired surface wall-jet-like effect. The response of the fluid to these forces requires some charged and neutral species interaction past complex configurations under conditions where transition and turbulence are dominant.
The standard monolayer designs shown in FIGS. 1 and 2 can produce discharge when several kilovolts are applied across the two electrodes separated by the layer of dielectric. This discharge induces body force in a small region resulting in a weak wall jet. Applications of polyphase RF power supply to the electrodes can nearly double the induced velocity as schematically shown in the right end of FIG. 2. However, that requires considerable power and an expensive power supply. Thus, the illustrated design requires a kHz RF power supply and can arc, making the discharge unstable.
These actuators operate at reasonable power consumption levels at lower speeds but lose performance at higher flow speeds, in which case a magnetic field is applied to induce additional Lorentz forces for effective control. This significantly increases the power requirement. Moreover, the RF transformers, along with the power supply needed to produce these discharges, are cumbersome and not suitable for many onboard applications.
Thus, the need exists for a plasma actuator design that can be applied at higher speeds and that can be used for onboard applications.